Conventionally, as a bonding material for mounting a semiconductor element and an electronic part other than the semiconductor element, a Sn—Pb solder material containing lead, and in particular, a Sn—Pb eutectic solder material having a 63Sn—37Pb eutectic composition (a composition of 63% Sn and 37% Pb by weight) have been generally used.
A mounted structure using a solder material as a bonding material is shown in FIGS. 4A and 4B.
FIG. 4A is a plan view while FIG. 4B is a cross-sectional view taken along A-AA in FIG. 4A. A portion of FIG. 4B is shown enlarged. Semiconductor packages 2 such as BGA/LGAs (Ball Grid Array/Land Grid Arrays) and chip parts 3 other than a semiconductor package are mounted to a substrate 1 by a solder 5.
As junctions of semiconductor packages 2a and 2b become more minute, cracks are more likely to occur during temperature cycle tests and drop tests. Therefore, a molding structure has been adopted in which spaces between the substrate 1 are molded with a molding resin 4 to provide reinforcement. However, there has never been a case where the chip parts 3 are molded with the molding resin 4.
Furthermore, in recent years, lower profiles of the semiconductor packages 2a and 2b have given rise to demands for increased mechanical strength of soldered sections and improved reliability characteristics such as thermal-shock strength in regards to mounting the packages 2 on the substrate 1.
In response to environmental problems, bonding materials are also undergoing a transition to solder materials that do not include lead, i.e., lead-free solder materials. An example of a lead-free solder having two types of metals as primary components thereof is a Sn—Ag solder that is a eutectic alloy material (Patent Document 1, Patent Document 2).
However, the melting point of a Sn—Ag solder is approximately 30 to 40° C. higher than the melting point of a Sn—Pb solder (approximately 183° C.) and, in association therewith, the soldering temperature becomes higher as compared with a case where a Sn—Pb solder is used.
Therefore, the use of a Sn—Ag solder may give rise to situations where a mounting temperature at which the chip parts 3 are mounted on the substrate 1 exceeds the allowable temperature limit of the chip parts 3, which may disadvantageously cause damage to the chip parts 3. Currently, such a case imposes cumbersome solder processing including performing soldering while fitting the chip parts 3 with a protective jig so that the temperature of the chip parts 3 whose allowable temperature limit is lower than the mounting temperature does not exceed the allowable temperature limit, soldering the chip parts 3 with a low allowable temperature limit in a post-process, and the like.
Moreover, in response to demands for smaller sizes and lower profiles in electronic products, the profile of the substrate 1 has been also lower. Therefore, as the mounting temperature becomes higher, warpage of the substrate 1 occurs which disadvantageously degrades the bonding quality of the junction between the substrate 1 and the semiconductor packages 2a and 2b as well as the bonding quality of the junction between the substrate 1 and the chip part 3.
In consideration thereof, for the purpose of reducing or preventing thermal damage to the chip part 3, a conductive adhesive whose curing temperature is relatively lower than the melting point of a lead-free solder and a Sn—Bi solder having a low melting point have been drawing attention as alternative materials to such a solder (Patent Document 3).
[Patent Document 1] Japanese Patent No. 3027441
[Patent Document 2] U.S. Pat. No. 5,520,752
[Patent Document 3] Japanese Patent Laid-Open No. 10-163605